A cathode ray tube, such as the well-known television tube, customarily consists of a panel or faceplate upon which the viewing screen is mounted, a neck portion within which an electron gun is mounted, and a funnel portion which separates the screen and gun. In operation, a large voltage drop is imposed between the screen and the electron gun, whereby electrons generated by the gun are impelled toward the screen. As a consequence, electric charges develop within the tube, and it has been the practice to provide an electrically conductive coating on the inside of the funnel portion. This conductive coating serves to drain off these charges and otherwise establish electrical contact between an electrical contact button in the tube wall and both the screen area and the gun area in the neck.
One early type of conductive coating is described in U.S. Pat. No. 2,699,510 which discloses the application of an enamel to the metal cone (now termed funnel) of a cathode ray tube. The operable enamels were required to have high melting points (800.degree.-1100.degree. C.). Where desired, the enamel, in fine particulate form, was mixed with about 5-10% by weight of graphite and the mixture applied to the metal cone by means of a liquid vehicle by spraying, immersion, or some other process. The single working example illustrated firing an enamel-graphite mixture on an iron core at 1100.degree. C. The use of graphite in amounts exceeding 10% was stated to adversely affect the mechanical resistance of the resultant layer.
The coating traditionally used has been referred to as a "dag" coating. This coating consists essentially of colloidal carbon suspended in an alkali metal silicate, in particular sodium silicate. The coating is applied by suspending colloidal carbon in an aqueous solution of alkali metal silicate, painting or spraying a very thin layer of the suspension over the inside surface of a tube funnel, and then drying to a thin adherent coating normally not over a mil in thickness.
The "dag" coating has been well received because of its suitable electrical characteristics, ease of application, and low material cost. Nevertheless, its use has not been without problems. In particular, this coating does not weather well, tending to form crystalline patches. These may differ markedly in expansion from the underlying glass, thus causing cracking during processing, and/or reaction with moisture resulting in alkaline attack on the glass. Also, there is a tendency for particle flaking to occur within the tube due to poor adherence of the coating. Such particles may lodge in the shadow mask of a color tube, may lodge on the screen, or may cause arcing in the neck portion.
Accordingly, efforts have been made to use other materials, but these have either been too expensive, or have themselves not provided adequate adherence and durability. The need for good durability and weathering characteristics becomes particularly critical when coated tubes must either be stored or transported under humid conditions before final assembly.
A variation in the traditional "dag" coating is disclosed in U.S. Pat. No. 3,108,906 which describes the application of graphite-containing, conducting coatings to the inner surface of the glass funnel portion of a cathode ray tube. The coating comprises an aqueous suspension containing potassium silicate, graphite, and ZnO. The suspension, containing particulate ZnO which does not dissolve therein, is permitted to dry on the glass surface with the potassium silicate acting as an adhesive. Such coatings are highly crystalline. Since the ZnO is not dissolved in the potassium silicate solution, it has no substantial effect upon the chemical properties thereof. Its presence influences only the physical properties of the overall coating in its inclusion as a filler. The patent does not specify operable limits of graphite additions but the two exemplary compositions provided contained approximately 67% and 56% by weight, respectively.